A multiplicity of methods are known which utilize the interaction of electromagnetic radiation in the infrared, visible and ultraviolet spectral ranges with matter to melt, evaporate, remove (ablate) material (U.S. Pat. No. 4,494,226), to induce phase transitions (U.S. Pat. No. 6,329,270) or to modify other physical or chemical material properties.
If the field of interaction between laser light and a workpiece is spatially shaped on the surface of the workpiece, e.g. by optical masks or by successive shifting of the laser focus, it is possible to successfully generate line- and area-type structures during processing; and three-dimensional structures can be achieved by layered removal as well as, in transparent media, by positioning the laser focus in the depth of the material (DE 100 06 081 A1).
Many of these methods require high power densities, which can be achieved in particular by application of pulsed laser radiation sources. By using laser pulses of short duration (a few nanoseconds), particularly efficient processing is achieved (U.S. Pat. No. 6,281,471). Interfering modifications of the workpiece outside the zone of interaction, which are caused by thermal effects, can be further reduced through an even shorter pulse duration (U.S. Pat. No. 6,150,630). It is thus possible, e.g. by means of ablation, to generate very fine structures, wherein the size of the material areas in which an interaction with the radiation occurs and those which experience no substantial modification as compared to their initial condition is given only by the size of the laser focus. The theoretical limit for the minimal structural dimensions is then given by the diffraction limit and, thus, ultimately by the wavelength of the laser radiation used. In particular, the use of laser pulses with pulse durations ranging from approximately 20 fs to 1,000 ps enables direct micro-processing of material (F. Korte et. al.: “Sub-diffraction limited structuring of solid targets with femtosecond laser pulses”, Optics Express 7, 2000, 41), which includes, in addition to technical uses, also medical uses, in particular in microsurgery. Moreover, apparatuses for generating spectrally broad-band laser pulses are widely applied as ultrashort-pulse lasers in research.
Two experimental papers by Stoian et al. (R. Stoian et al.:“Laser ablation of dielectrics with temporally shaped femtosecond pulses”, Appl. Phys. Lett. 80, 2002, 353; R. Stoian et al.: “Ultrafast laser material processing using dynamic temporal pulse shaping”, RIKEN Review 50,2003) disclose how the structuring process in laser ablation can be optimized as regards reducing residual damage by means of temporal shaped laser pulses. For this purpose, different pulse trains were generated by means of phase modulation, and the advantages of their use as compared to unshaped laser pulses of the laser system used in ablative laser boring of selected materials were experimentally shown under vacuum conditions. In doing so, unshaped and shaped laser pulses were respectively directed onto the surface of a-SiO2 and Al2O3 for comparison and the result of processing was then visually analyzed with the help of a light microscope.
In laser material processing of composite materials, it is possible to select the amplitude spectrum of the laser pulses used such that material-selective processing is possible. The selection of a suitable laser under the aspect of adapting the laser wavelength to the material to be processed is a known method (e.g. U.S. Pat. Nos. 5,948,214, 5,948,214, 4,399,345, and 5,569,398). However, the physical-technical properties of the object to be processed may change during the processing operation, e.g. by material heating. In particular, changes of the absorption characteristic of composites limit material selectivity in the processing operation (U.S. Pat. No. 6,281,471), because an adequate change of the laser wavelength is hardly possible with the lasers used for material processing.
Therefore, a method had to be provided making processing effects possible with as little effort as possible and in a manner as flexible and as universally applicable as possible, said effects respectively being specifically determinable and adaptable with regard to the processing task and the course of the process.